1. Field of the Invention
The present invention relates to a driver circuit for driving into ON and OFF conditions of a large capacity switching element, such as a gate turnoff thyristor (GTO), a power transistor, a static inductive thyristor (SITH), etc., and more particularly to a driver circuit for supplying a large current at the starting time of the ON-condition, which is so called overdrive function.
2. Related Art Statement
As a driver circuit for a switching element and more especially such a circuit having overdrive function, a basic circuit shown in FIG. 7 has been known.
Namely, FIG. 7 shows in general a conventional embodiment for a basic driver circuit having such an overdrive function. In FIG. 7, 1 indicates a switching element to be controlled, 2, 3 and 4 are D.C. voltage sources, 5 and 6 are resistors for current limiting, 7 a switching element for overdriving, 8 a switching element for continuous ON condition, 9 an inverter circuit for signal polarity reversing, 10 a monostable multivibrator circuit for deciding the overdrive period, and 11 is a switching element for the OFF condition.
When a control signal S.sub.c, (in this case, low level signal (L) as shown in the drawing), is applied, the switching element 11 is turned off and at the same time, the inverter circuit 9 and the monostable multivibrator circuit 10 operate and causing the switching elements 8 and 7 to be turned on. In this situation, a large overdrive current is applied from the D.C. voltage source 4 via the resistor 5 to the controlling terminal of the controlled switching element 1, and at the same time, a continuous ON current is supplied from the D.C. source 2 via the resistor 6 to said controlling terminal. The switching element 7 keeps the ON condition for a period decided by the monostable multivibrator 10, and after lapse of said period i.e. after it is off, a current supplied only from the switching element 8.
When the control signal S.sub.c disappears i.e. a high level signal H appears at the input terminal, the switching element 8 is turned off, and at the same time the switching element 11 is turned on. Then, a reverse polarity from the D.C. voltage source 3 is applied to the controlling terminal of the controlled switching element 1 and this element 1 is turned off by an OFF current passing therethrough.
However, in such a known device as shown in FIG. 7, the resistance values of the resistors 5 and 6 are decided by considering the minimum voltage value in the voltage variation ranges of the D.C. voltage sources 4 and 2, the maximum values of the ON time voltage drop of the switching elements 7 and 8, and the maximum value of the voltage drop between controlling terminals of the controlled switching element 1 so that when considering a case in which these values are normal, the capacity of the driving current of the controlled switching element, becomes unnecessarily large.
Accordingly, such a known device has disadvantages and problems in that the losses in the resistors 5 and 6 are too large and that the capacity of D.C. voltage sources becomes too large since three circuits of the D.C. voltage source are required.